Double filament lamps are shown, by way of example, in U.S. Pat. Nos. 2,005,194, 3,493,806, 3,777,207, 3,784,861, 4,074,167, 4,361,779 and 4,361,780. A multi-filament lamp is described in U.S. Pat. No. 4,288,844 which, in some embodiments described therein, includes more than two filaments. In particular, this patent illustrates a lamp wherein two or more filaments are arranged perpendicular to the optical axis to the light. With such an arrangement, redundancy is not provided because one filament is free to fall directly onto the other upon failure thereof. Similarly, in the other patents referred to herein, redundancy (isolation from filament-to-filament) is not provided and thus at failure of any one filament, there tends to be interference with the non-failed filament.
In the past, separate lamp envelopes have been used in an effort to provide desired redundancy or isolation. However, this has typically resulted in a more complex and thus more costly overall lamp construction.